1. Field of the Invention
The present invention relates generally to power converters, and more particularly to switchable power converters for multiple level input line power factor correction.
2. Background of the Invention
FIG. 1 illustrates a conventional power converter circuit operating as a high frequency electronic ballast for multiple gas discharge lamps 190. Referring to FIG. 1, the power converter circuit basically comprises two stages. The front end is a boost converter 100 for universal line power factor correction and universal line voltage regulation. The boost converter 100 is primarily comprised of power switch 102, inductor 104, diode 106, DC bus capacitor 108 and power factor correction (PFC) control integrated circuit (IC) 109.
The back end is a typical voltage-fed half-bridge inverter 140 loaded with the lamps 190 through a resonant tank circuit comprised of a capacitor 152 and inductor 154, along with the magnetizing inductance associated with output transformer 156. The half-bridge inverter is primarily comprised of power switches 148 and 150 which are controlled by a high voltage control integrated circuit (IC) 151, as known in the art.
The boost converter of FIG. 1 is ideal for providing a DC bus voltage 112 of 450 VDC output across capacitor 108 for input voltages of 120V/277V AC at Vin. The relatively high DC output voltage level is because the intrinsic topology of the boost converter causes the DC bus voltage 112 to be greater than the peak value of the input line voltage.
However, some applications require a lower DC bus voltage, for example 225V DC. In those applications a flyback converter is better suited, since the flyback converter is capable of generating the relatively low DC bus voltage of 225V DC from an input voltage of 120V/277VAC. The flyback converter, however, has several drawbacks, including higher component stresses, lower overall efficiency, larger component sizes, and severely large electromagnetic interference (EMI) conditions.
Alternatively, a single-ended primary inductance converter (SEPIC) may be employed. The SEPIC is capable of producing an intermediate DC output voltage, such as 225V DC. While the SEPIC shares some of the drawbacks of the flyback converter, such as higher component stress, lower overall efficiency and a resulting larger size due to additional DC blocking capacitors, the SEPIC enjoys improved EMI conditions. This is because the SEPIC input section is similar in configuration to the boost converter input section.
It is a characteristic of both the flyback and SEPIC converters that the highest losses occur at the lowest input line voltage and the highest voltage stresses occur at the highest input line voltage over a universal input line voltage range. Among the flyback, SEPIC and boost converters, the boost converter exhibits the highest efficiency and lowest voltage stresses. However, as discussed above, the boost converter is only viable for use at lower input line voltages in the case of 225V DC output bus voltage specifications.
To overcome the above stated disadvantages, a switchable power converter was disclosed in U.S. patent application Ser. No. 09/716,698, entitled xe2x80x9cSWITCHABLE POWER CONVERTER FOR MULTIPLE LEVEL INPUT LINE POWER FACTOR CORRECTIONxe2x80x9d for Chin Chang, Adan Hernandez and Gert Bruning, filed on Nov. 20, 2000, now U.S. Pat. No. 6,373,725; the contents of which are hereby incorporated by reference. The application discloses a switchable power converter that advantageously switches between a boost converter circuit topology, for low input line voltages, and either a flyback or SEPIC converter circuit topology, for high input line voltages, to provide an intermediate DC output voltage level, such as 225V DC, over a range of input line voltage levels. However, depending on the switch position, certain configurations may result in an open-ended terminal of an inductor, which may create EMI conditions due to antenna effect. Additionally, the boost converter topology results in a large input current ripple requiring a large EMI filter.
Therefore, a further switchable power converter is needed that advantageously switches between a boost converter circuit topology, for low input line voltages, and a SEPIC converter circuit topology, for high input line voltages, that reduces the antenna effect of the circuit and provides a current steering ripple effect in both the booster and SEPIC converter circuit topologies.
It is therefore an object of the present invention to provide a switchable power converter including a coupled inductor and a configuration switch arranged to prevent an open-ended terminal of the coupled inductor in any state of the configuration switch.
It is another object of the present invention to provide a switchable power converter having improved efficiency and reduced stresses over a range of input line voltages.
It is a further object of the present invention to provide a switchable power converter having a reduced antenna effect for improved EMI conditions in either of the switchable configurations.
It is a further object of the present invention to provide a switchable power converter having a current ripple steering effect for improved EMI conditions in either of the switchable configurations.
To achieve the above objects, a switchable power converter in accordance with the present invention includes an input section that receives an AC input voltage- and rectifies the AC input voltage and a switchable converter section operative to receive the rectified AC input voltage and convert the rectified AC input voltage to an intermediate DC output voltage. The switchable converter section includes at least one configuration switch operative to switch the switchable converter section between a boost converter topology, for low input line voltages, and a SEPIC converter circuit topology, for high input line voltages, and also includes a coupled inductor. The configuration switch eliminates the open-ended terminal in a load inductor of the coupled inductor thereby reducing any antenna effect. Additionally, the coupled inductor achieves a current ripple steering effect in the boost converter topology, similar to that of the SEPIC converter topology, resulting in a smaller input current ripple requiring a smaller EMI filter. The configuration switch may be a relay based mechanical switch or a solid state switch, for example.